c# - View isn't showing in MVVM -

i'm trying use mvvm design pattern. searched , read many mvvm documents , examples, it's difficult me.

i want navigate , show view main window code isn't working.

without <window.resources> ~ </window.resources> part, contentcontrol elements shows text 'myproject.viewmodel.authviewmodel'.

but if put <window.resources> part, nullreferenceexception: object reference not set instance of object. @ myproject.helper.delegatecommand.canexecute(object parameter) error occurs

what wrong?

mainviewmodel.cs

public class mainviewmodel : viewmodelbase {     private icommand _changeviewcommand;     private viewmodelbase _currentviewmodel;     private list<viewmodelbase> _viewmodels;      public mainviewmodel(){         viewmodels.add(new authviewmodel());         currentviewmodel = viewmodels[0];        }      #region commands     public icommand changeviewcommand     {                 {             if (_changeviewcommand == null)             {                 _changeviewcommand = new delegatecommand(                     param => changeviewmodel((viewmodelbase)param),                     param => param viewmodelbase                 );             }             return _changeviewcommand;         }     }     #endregion      #region properties     public list<viewmodelbase> viewmodels     {                 {             if (_viewmodels == null)                 _viewmodels = new list<viewmodelbase>();              return _viewmodels;         }     }      public viewmodelbase currentviewmodel     {                 {             return _currentviewmodel;         }         set         {             if(_currentviewmodel != value)             {                 _currentviewmodel = value;                 raisepropertychanged("currentviewmodel");             }         }     }     #endregion      private void changeviewmodel(viewmodelbase _viewmodel)     {         if (_viewmodel != null && !viewmodels.contains(_viewmodel))             viewmodels.add(_viewmodel);          currentviewmodel = viewmodels.firstordefault(_vm => _vm == _viewmodel);     } 

mainview.xaml

<window x:class="myproject.view.mainview"         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"         xmlns:v="clr-namespace:myproject.view"         xmlns:vm="clr-namespace:myproject.viewmodel"         xmlns:mp="clr-namespace:myproject"         title="myproject" height="630" width="460" windowstartuplocation="centerscreen" snapstodevicepixels="true">     <window.resources>         <datatemplate datatype="{x:type vm:mainviewviewmodel}">             <v:beamview />         </datatemplate>         <datatemplate datatype="{x:type vm:authviewmodel}">             <v:authview/>         </datatemplate>     </window.resources>      <window.datacontext>         <vm:mainviewmodel/>     </window.datacontext>      <grid>         <contentcontrol content="{binding currentviewmodel}" horizontalcontentalignment="stretch" verticalcontentalignment="stretch" verticalalignment="stretch" horizontalalignment="stretch"/>     </grid>  </window> 

dayum. <datetemplate> didn't cause problem.

i wrote wrong code @ delegatecommand class

public override bool canexecute(object parameter) {     if (parameter != null) // hell ?         return true;     return _canexecute(parameter); } 

and fixed it

public override bool canexecute(object parameter) {     if (parameter == null)         return true;     return _canexecute(parameter); }